Allochronic speciation (also known as allochronic isolation, or temporal isolation) is a form of speciation (specifically ecological speciation) arising from reproductive isolation that occurs due to a change in breeding time that reduces or eliminates gene flow between two populations of a species. The term allochrony is used to describe the general ecological phenomenon of the differences in phenology that arise between two or more species—speciation caused by allochrony is effectively allochronic speciation.

Environmental changes acting on a species population or populations can drive isolation. An important form of isolation is when populations are separated, not geographically, but temporally (by time). Genetic changes (mutations) over time can cause the two populations to differ—notably in phenology (events in a species life dictated by time such as breeding seasons); exhibiting unique phenotypes (the observable characteristics or traits of an organism).

Scientists have developed models to explain how this process occurs and how it is detected in natural populations. A wealth of studies exist regarding species in allochrony, with a select few that strongly suggest species are speciating or already have speciated as a direct consequence of this mode of isolation.

Speciation ultimately results due to the reproductive isolation between two populations. This can happen in a multitude of ways, a common mode of which is known as allopatric speciation. The geographic mode, where two species become physically isolated and unable to interbreed, allows for selection to act on both populations independently. Over time, this gives rise to a new species.^[1]: 86 Allochronic speciation is a form of isolation that can involve allopatry; however, it is not required.^[1]: 208

Allochrony can involve a number of factors that induce the formation of a new species. Organisms have evolved various reproductive strategies (e.g. semelparity and iteroparity, single or multiple reproductive cycles in a lifetime) that can result in different outcomes for allochrony. Many organisms also breed at different times of the day, different seasons in the year, and even over multiple years or decades.^[2] Seasonal breeding in animals is a common occurrence as well as spawning (in aquatic animals) times.^[1]: 202 In plants, breeding in regards to time could involve the receptivity of the stigma (the female part of the flower) to accepting sperm, periods of pollen release (such as in conifer trees where male cones disperse pollen relying on wind to direct pollen to female cones), or the overall timing of flowering (based on possible environmental cues such as moisture levels, soil type or quality, temperature, or photoperiod).^{[1]: 202–206} Even migratory patterns can play a role, as species may become isolated due to migrating at different times and to different locations.^[3] Climate change is considered to have a significant impact on allochrony—in particular, seasonal breeding species.^[2] Modeling changes in species breeding patterns due to climate as well as understanding the genetic mechanisms that control it has proven to be important.^[2]

Because of these many factors, slight to major changes in phenology can drive divergence between two populations. For example, a species with multiple breeding seasons in a year may shift those times depending on external conditions such as temperature or predation. In the event the populations (either allopatrically or sympatrically distributed, started breeding at different times, it would prevent members of each population from exchanging genes with one another. Over time, if genes are not exchanged, genetic differences arise in each population. If natural selection acts strongly on the two populations, they may become reproductively isolated, unable to reproduce viable, fertile offspring.^{[1]: 202–210}

For allochronic speciation to be considered to have actually occurred, the model necessitates three major requirements:^[2]

• Phylogenetic analysis must indicate that the two taxa in question are incipient species or clearly sister taxa.
• Breeding timing is required to be genetically-based (heritable) as opposed to changeable throughout life (phenotypic plasticity.
• The source of divergence can be determined to be explicitly allochrony and not the result of reinforcement or other evolutionary mechanisms.

Allochrony is thought to evolve more easily the greater the heritability of reproductive timing—that is, the greater the link between genes and the timing of reproduction—the more likely speciation will occur.^[4] Allochrony can be non-genetic;^[1]: 203 however genetic factors must be involved for isolation to lead to complete reproductive isolation and subsequent speciation. The time frames involving allochrony are typically divided into three categories (prevalence in nature as well as examples are provided alongside each category):^[2]

• Daily (considered to be common), examples include stony corals such as Acropora^[5] or Orbicella.^[6]
• Seasonally (considered to be the most common), seasonal breeding times often coincide with winter, spring, fall, or summer; examples include salmon breeding runs such as in sockeye salmon.^[7]
• Yearly (considered rarer), examples include periodical cicadas^[8] and bamboo,^[9] both of which reproduce within a scale of decades.

Because allochronic speciation can occur in conjunction with other modes and forms of speciation, researchers must attempt to determine if the initial stages of isolation were the result of allochrony. The speciation continuum of allopatry, parapatry, and sympatry have all been implicated in studies of temporal isolation.^[1]: 206 Allochrony can also facilitate reinforcement after secondary contact.^[2] The frequency of allochronic speciation is thought to common but understudied as allochrony is widespread in nature.^[14]

Testing whether or not allochrony prevents gene flow can be difficult due to the multitude of unknown variables in wild populations and the inability to replicate and manipulate it in laboratory settings.^[1]: 203 Producing viable, and fertile offspring (or the lack thereof) is not always possible; fortunately, lake of mate tests do not necessarily indicate temporal isolation is not at play.^[1]: 203 As stated prior, one of the necessary criteria is that the species in question must be sister taxa (or demonstrably incipient). This means that accurate phylogenies are vital to determining the initial stage of a speciation event.^[1]: 203

Despite the multitude of studies, it is not always evident whether allochrony is the sole driver of speciation or if other factors acting simultaneously are responsible.^[2] This can be more challenging when speciation has already occurred (in that the taxa in question are reproductively isolated and no longer incipient).^[2] Determining how important allochrony is as a historical cause of speciation can be tested by: 1) comparative studies that show the young pairs of sister taxa are temporally isolated and 2) testing cases of incipient species in sympatry where reproductive isolation is incomplete without temporal isolation.^[1]: 206

Determining if allochrony is the source of divergence require a key pattern to be measured: isolation (and subsequently speciation) should correlate with a decrease in overlapping breeding times.^[2] This pattern indicates that daily allochrony is more prone to gene flow (closeness of breeding times can allow accidental intermixing of populations) while yearly allochrony is the least prone to gene flow (accidental intermixing is rare if large time frames exist between mating periods).^[2]

The following table documents cases of allochronic speciation. Varying degrees of certainty exist as not all cases strongly meet the three primary criteria necessitated by allochronic speciation. Species marked with an asterisk (*) indicate stronger confidence assessed by Rebecca Taylor and Vicki Friesen (2017).^[2]